docs: def raise_failure_exception(self, expect_or_allow='Allowed'):
          """Raises a ``MockExpectationError`` with a useful message.
  
          :raise: ``MockExpectationError``
          """
  
          raise MockExpectationError(
              "{} '{}' to be called {}on {!r} with {}, but was not. ({}:{})".format(
                  expect_or_allow,
                  self._method_name,
                  self._call_counter.error_string(),
                  self._target.obj,
                  self._expected_argument_string(),
                  self._caller.filename,
                  self._caller.lineno,
              )
          )